This invention relates to producing a high emittance coating on aluminum or its alloys, and is particularly concerned with a novel anodizing process for aluminum or its alloys to achieve a coating having high infrared emittance and also low solar absorptance, and the product so produced.
In space, there is no atmosphere to conduct heat to or from a spacecraft. Therefore, all heat gain or loss must be by radiation. Radiation is accomplished through the use of thermal control surfaces which can absorb solar radiation and emit radiation to space. These surfaces have a range of desirable values for solar absorptivity (.alpha.) and infrared emissivity (.epsilon.). For surfaces such as the radiators, it is important to absorb as little solar radiation as possible (low .alpha.) while radiating as much heat as possible to space (high .epsilon.).
The .alpha. and .epsilon. properties of the thermal control surfaces must be stable to maintain the temperatures of the spacecraft in the range required for effective operation. However, spacecraft which are in orbit near the earth (commonly called the low earth orbit or LEO) experience a hostile space environment consisting of atomic oxygen, ultraviolet radiation, charged particles, and contamination from other spacecraft components. These factors have been known to degrade the optical properties of spacecraft thermal control surfaces.
The development of a suitable long-life thermal control coating is therefore essential for the longevity and integrity of spacecraft structures. This coating must also be economical and easy to handle and apply to structures. Common radiator coatings include inorganic white paints, silver-coated Teflon films, and silver-coated quartz tiles and anodic coatings. Although organic coatings such as silicone and fluorocarbon base coatings, can provide the desired optical properties, they are attacked and erode in the LEO environment. The quartz tiles have been very labor intensive to install particularly for the complex geometry of most spacecraft and are quite fragile. Inorganic paints can achieve high emissivity but weigh more than anodic coatings and Teflon is not resistant to the LEO environment. Anodic coatings of aluminum are one of the most attractive thermal coating systems because of the light weight of the anodic coating, it is integral with the aluminum substrate, it does not spall or chip even from micrometeoroid/debris impact, and is completely resistant to erosion from atomic oxygen. In addition relatively high emissivities can be obtained.
Anodizing is an electrolytic process that produces an oxide film on the surface of a metal. When aluminum is anodized in a sulfuric acid electrolyte, a porous film of aluminum oxide is formed on the surface of the part. Anodized 5657 aluminum represents a promising candidate for the thermal control coating of the radiators. It has a low .alpha. and a relatively high .epsilon.; typically, .alpha.=0.2 and .epsilon.=0.85 for a 0.001 inch thick coating. However, a higher emissivity is more desirable for spacecraft thermal surfaces such as the radiators. A more efficient radiator results in less radiator surface required for the task, hence less weight. For example, an increase of 1% in the emissivity can reduce the size of a radiator panel by 1%. Additional drawbacks associated with anodic coatings is the higher solar absorptance .alpha. with some aluminum alloys than desired and the increase in solar absorptance that occurs with LEO space exposure.
Representative of the prior art is Gilliland et al U.S. Pat. No. 4,397,716 which discloses anodizing aluminum surfaces in chromic acid as the anodizing electrolyte to obtain an anodized coating adapted to be exposed to solar radiation and having a thermal emittance in the range of 0.10 to 0.72 and a solar absorptance in the range of 0.2 to 0.4. However a higher thermal emittance is required for more efficient spacecraft thermal surfaces, as noted above. Further, chromic acid anodizing produces thin rather amorphous oxide coatings whereas sulfuric acid anodized coatings are much thicker and exhibit a columnar crystalline structure. In addition, chromic acid anodizing produces a matte surface finish with low infrared emittance, as well as high solar absorptance, whereas sulfuric acid anodizing yields a transparent and semispecular coating with a higher infrared emittance and a lower solar absorptance.
Accordingly, one object of the invention is the provision of procedure for producing a high emittance coating on aluminum or its alloys.
Another object is to provide novel anodizing procedure for aluminum or its alloys, so as to result in an anodized coating having high infrared emissivity, and also low solar absorptivity.
A still further object is the provision of a high emittance anodized coating on aluminum or an alloy thereof.
Further objects and advantages will appear hereinafter.